Address hbase-shell commands with unintentional return values

There are a number of commands in hbase-shell that return values that are undocumented or otherwise do not makes sense.

The commands that will be addressed by this ticket were found in a survey of all the hbase shell commands, https://issues.apache.org/jira/browse/HBASE-24672.

Subtasks:

• Fix commands that return the number of printed lines (usually the number 1).
  • Affected commands (culprits): is_disabled, balancer, normalize, normalizer_switch, normalizer_enabled, catalogjanitor_switch, catalogjanitor_enabled, cleaner_chore_switch, cleaner_chore_enabled, splitormerge_switch, splitormerge_enabled, clear_deadservers, clear_block_cache
  • I am updating these 13 commands to either return a value that makes sense or return nil if a sensible return value doesn't exist.
• Fix commands that return true or false as a String
  • Affected commands (culprits): balance_switch, snapshot_cleanup_switch, enable_rpc_throttle, disable_rpc_throttle, enable_exceed_throttle_quota, disable_exceed_throttle_quota
  • I am updating these 6 commands to return ruby's TrueClass or FalseClass rather than a String